fix(runtime tests): treat write's better

- unwrap on writeln!()
- make sure "all" [0] is written in capture

Signed-off-by: Charlotte Meyer <dev@buffet.sh>

crates/oyster_runtime/tests/it/builtins/mod.rs

@@ -1,6 +1,10 @@
 mod math;
 
-use std::{borrow::Cow, ffi::OsStr, io::Write};
+use std::{
+    borrow::Cow,
+    ffi::OsStr,
+    io::{self, Write},
+};
 
 use expect_test::{expect, Expect};
 use oyster_builtin_proc::builtin;
@@ -11,8 +15,7 @@ use crate::collect_output;
 
 #[builtin(description = "test builtin")]
 fn test_builtin(_: &mut Shell, _: &[Cow<OsStr>]) {
-    // XXX: this is a workaround around libtest's use of io::set_output_capture
-    let _ = write!(std::io::stdout(), "this is a test\n");
+    writeln!(io::stdout(), "this is a test").unwrap();
 }
 
 fn check(ast: &ast::Code, expect: Expect) {

crates/oyster_runtime/tests/it/main.rs

@@ -50,7 +50,7 @@ where
     let master = unsafe { File::from_raw_fd(master) };
     let mut slave = unsafe { File::from_raw_fd(slave) };
 
-    slave.write(&[0]).unwrap();
+    slave.write_all(&[0]).unwrap();
     let mut r = BufReader::new(master);
     let mut buf = vec![];
     r.read_until(0, &mut buf).unwrap();